Boris Johnson is expected to comply with a London Assembly order to explain his links to a US businesswoman.
Len Duvall, chairman of City Hall’s oversight committee, said: “We are going to have something this evening from Downing Street.”
The PM is facing questions about his friendship with Jennifer Arcuri when he was London mayor.
He has been accused of failing to declare a conflict of interest, but has said he acted properly at all times.
Mr Johnson had been given until Tuesday to provide details of contacts with Ms Arcuri.
Mr Duvall said: “We have had some fun and games today arguing about when is the deadline, but we finally have an announcement that they are going to comply, and we are going to get something this evening from Downing Street. I hope it is comprehensive and I hope it provides answers.
“The allegations are serious, I hope the prime minister is treating them seriously.”
He said the assembly’s powers to take action against Mr Johnson, if he was found to have breached its code of conduct, were limited because he was no longer mayor of London.
But it could still summon the prime minister to appear before the oversight committee to answer further questions about his contacts with Ms Arcuri.